Electronic apparatuses such as portable communications terminals of mobile communications system which use high frequencies (such as microwave band) employ a large number of electronic components constituted from dielectric materials.
In transmission paths, resonating circuits and filter circuits, for example, microstrip lines shown in FIG. 13 are used. The microstrip line is constituted from a ground surface (ground line) 4, an electrically conductive layer 1 (circuit pattern) and a dielectric layer 2 interposed therebetween. Characteristic impedance of the microstrip line is determined by width W of the electrically conductive layer 1, thickness t, distance between electrical conductors, thickness d of the dielectric layer 2 and relative dielectric constant εr thereof. A dielectric resonator antenna shown in FIG. 14 is constituted by interposing the dielectric layer 2 between an antenna pattern 14 and the ground surface 4, and radiates electromagnetic wave when a high-frequency signal is supplied to an antenna feeding point 6.
There is a method proposed for changing the electrical characteristics of a component by changing the dielectric constant of the dielectric member in a microstrip line or a dielectric resonator antenna which uses a dielectric material. For example, a characteristic impedance Z0 of the microstrip line shown in FIG. 13 can be given by the following equation.When W/d≦1.0:
            Z      ⁢                          ⁢      0        =                            60                                    ɛ                        ⁢                                                  ⁢            e                          ·        1            ⁢                          ⁢              n        ⁡                  [                                                    8                ⁢                                                                  ⁢                d                            W                        +                          W                              4                ⁢                                                                  ⁢                d                                              ]                                ɛ      ⁢                          ⁢      e        =                                        ɛ            ⁢                                                  ⁢            r                    +          1                2            +                                                  ɛ              ⁢                                                          ⁢              r                        -            1                    2                ·                  [                                    1                                                1                  +                                      12                    ⁢                                                                                  ⁢                                          d                      /                      W                                                                                            +                          0.04              ⁢                                                (                                      1                    -                                          W                      d                                                        )                                2                                              ]                    When W/d≧1.0:
            Z      ⁢                          ⁢      0        =                  120                              ɛ                    ⁢                                          ⁢          e                    ·                        [                                    W              d                        +            1.393            +            0.667            +                          1              ⁢                                                          ⁢                              n                ⁡                                  (                                                            W                      d                                        +                    1.444                                    )                                                              ]                          -          1                                ɛ      ⁢                          ⁢      e        =                                        ɛ            ⁢                                                  ⁢            r                    +          1                2            +                                                  ɛ              ⁢                                                          ⁢              r                        -            1                    2                ·                  1                                    1              +                              12                ⁢                                                                  ⁢                                  d                  /                  W                                                                        
Thus characteristic impedance Z0 of the microstrip line is determined by three variables of d (thickness) which is a geometrical parameter, W (line width) and dielectric constant ε (or relative dielectric constant εr) of the dielectric material. It is in practice to change the dielectric constant of the dielectric material, among these parameters, so as to control the electrical characteristics of the microstrip line.
There have been two conventional methods for controlling the dielectric constant. One is to change the voltage and temperature of a solid dielectric material thereby to change the dielectric constant, and the other is to apply a voltage to a liquid crystal thereby to change the dielectric constant. Of the two methods, one that employs liquid crystal as the dielectric material will be described below.
FIG. 15 shows an example of the constitution of a microstrip line of the prior art where the dielectric constant of a dielectric material is changed. In FIG. 15, direction of polarization of a liquid crystal dielectric layer 7 with no voltage applied thereto is determined by the direction of a rubbing surface 3 (direction of orientation). When a voltage is applied from a dielectric constant control voltage source 35 to the liquid crystal dielectric layer 7, orientation of the liquid crystal molecules within the liquid crystal dielectric layer 7 changes due to the influence of the electric field generated by the voltage, so that the dielectric constant changes. Thus dielectric constant is changed so as to obtain the desired characteristics, by applying the voltage to the liquid crystal dielectric layer 7.
It should be noted that a prior art technology aimed at providing an antenna apparatus which enables operation over a broader frequency band by making the resonant frequency of the antenna variable (refer to, for example, Patent Document 1). The antenna apparatus comprises an antenna and a radio transceiver which feeds transmission signal to and receives reception signal from the antenna, wherein the antenna is provided with a dielectric member of which relative dielectric constant changes in response to a frequency control voltage Ec, while the frequency control voltage Ec applied to the dielectric member is controlled.
Patent Document 1: Japanese Unexamined Patent Publication, First Publication No. H11-154821